Imaging devices, such as optical scanners, are well-known in the art and produce machine-readable image data signals that are representative of a scanned object, such as a photograph or a page of printed text. In a typical scanner application, the image data signals produced by an optical scanner may be used by a personal computer to reproduce an image of the scanned object on a suitable display device, such as a CRT or a printer.
A hand-held or portable optical scanner is an optical scanner which is designed to be moved by hand across the object being scanned. The hand-held scanner may be connected directly to a separate computer by a data cable. If so, the data signals produced by the hand-held scanner may be transferred to the separate computer "on the fly," i.e., as the image data are collected. Alternatively, the hand-scanner may include an on-board data storage system for storing the image data. The image data may then be downloaded to a separate computer after the scanning operation by any convenient means, such as via a cable or an optical infrared data link.
Hand-held or portable optical scanners are well-known in the art and various components thereof are disclosed in U.S. Pat. No. 5,552,597 of McConica for "Hand-Held Scanner having Adjustable Light Path", U.S. Pat. No. 5,586,212 of McConica, et al., for "Optical Wave Guide for Hand-Held Scanner," U.S. Pat. No. 5,381,020 of Kochis, et al., for "Hand-Held Optical Scanner with Onboard Battery Recharging Assembly," and U.S. Pat. No. 5,306,908 of McConica, et al., for "Manually Operated Hand-Held Optical Scanner with Tactile Speed Control Assembly," all of which are hereby incorporated by reference for all that they disclose.
A typical hand-held optical scanner may include an illumination system and an optical system. The illumination system illuminates the object and the optical system collects light reflected by the illuminated object and focuses a small area of the illuminated object, usually referred to as a "scan region" or "scan line," onto the surface of a photosensitive detector positioned within the scanner. Image data representative of the entire object then may be obtained by sweeping the illuminated scan line across the entire object, usually by moving the hand-held scanner with respect to the object. By way of example, the illumination system may include a white light source (e.g., a fluorescent or incandescent lamp or an array of light emitting diodes (LEDs)). The optical system may include a lens and/or mirror assembly to focus the image of the illuminated scan line onto the surface of the detector.
The photosensitive detector used to detect the image light focused thereon by the optical system may be a charge-coupled device (CCD), although other devices may be used. A typical CCD may comprise an array of individual cells or "pixels" each of which collects or builds-up an electrical charge in response to exposure to light. Since the quantity of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on an image focused thereon. In a typical scanner application, the charge built up in each of the CCD cells or pixels is measured and then discharged at regular intervals, known as sampling intervals, which may be about 5 milliseconds or so for a typical scanner, although other sampling intervals may be used.
The term "image light" as used herein refers to the light that is focused onto the surface of the detector array by the optical system. Depending on the type of scanner and the type of document, the image light may be reflected from the document or object being scanned or it may be transmitted through the object or document. The image light may be converted into digital signals in essentially three steps. First, each pixel in the CCD detector converts the light it receives into an electric charge. Second, the charges from the pixels are converted into analog voltages by an analog amplifier. Finally, the analog voltages are digitized by an analog-to-digital (A/D) converter. The digital signals then may be processed and/or stored as desired.
One factor that affects the overall quality of the scanned image is the tonal resolution of the scanner. The tonal resolution is a measure of the number of different intensity levels (i.e., grayscale tones) that can be detected and recorded by the scanner. For example, an image scanner utilizing an 8-bit analog-to-digital (A/D) converter to convert the analog signals from the detector array into corresponding digital signals is capable of representing 2.sup.8 or 256 different intensity levels. The A/D converter returns a maximum value of 255 for a maximum intensity or "white" pixel and a minimum value of 0 for a minimum intensity or "black" pixel.
A scanner's "white intercept" or "white point," is the intensity of image light that results in a maximum value (i.e., full scale) output from the A/D converter of the scanner. Similarly, the "black intercept" or "black point" is the intensity of the image light (neglecting noise) at which the A/D converter returns a value of zero. Therefore, the bit depth of the A/D converter and the difference between the white and black intercepts determines the tonal resolution of the scanner.
On problem associated with scanner apparatus of the type described above relates to providing substantially equal illumination along the entire length of the scan line. Referring now to FIG. 1, consider, for example, an illuminated scan line S on an object O, image light from which is focused onto the surface of a detector D by a lens assembly L. In order for the detector D to capture the desired detail of the object O along the entire length of the scan line S, all portions of the illuminated object O bounded by the scan line S should equally illuminate the surface of the detector D. Unfortunately, however, the illumination, i.e., the density of luminous flux, on the surface of the detector D varies inversely with the square of the distance from the illuminated object O and directly with the cosine of the angle .theta. between the luminous flux and the normal N to the surface of the detector D. Consequently, the illumination on the surface of the detector D tends to decrease towards each end, thereby reducing the effective sensitivity of the detector D for points that lay near each end of the scan line.
Another problem commonly associated with a scanner of the type described above relates to the gradual degradation over time of the light source used to illuminate the scan line. Generally speaking, the light output of most of the commonly used light sources tends to decrease with age. If peak image quality is to be maintained, it is necessary compensate for the reduced light output.